Who knew Central Park is temporary home to hundreds of species of migratory birds? And that their devoted watchers — who prefer to be known as birders — include novelist Jonathan Franzen, author of “The Corrections’’?

Jeffrey Kimball’s charming, hourlong documentary, which will air on HBO later this year, is an engrossing tour of this little-known subculture and unlikely habitat in the center of densely populated Manhattan.

Beautifully photographed over the four seasons — including Christmas, for the park’s century-old bird census — “Birders: The Central Park Effect’’ is full of grace notes.

Not the least of them is the vibrant 70-something Starr Saphir, who continues to offer the guided bird-walks she’s given for two decades, even after being diagnosed with terminal breast cancer a few years ago.